And before someone says this doesn't belong in Religious Studies, I want to explain that to me, everything I learn about our origins is highly-interconnected with my faith and relationship with the Lord. One thing I'm realizing is that, the more open I am to the possibility of some Scriptures (such as the creation account), being more allegorical than literal (though every bit as packed with truth), the more evolutionary theory makes sense and doesn't seem to contradict my Christian faith.
Anyhow, I've recently wondered a lot about one key difference between other mammals and humans: other mammals mate when females are in heat (and it's very clear when other female mammal are ovulating), whereas humans mate much more frequently (and it's less clear when human females are ovulating).
Then today I stumbled across the above chapter I linked to on the internet (chapter 3 of The Five Billion Year Story
) -- and I was like, wow! -- this totally makes it so clear. At the point when chimpanzees realized it was beneficial to be able to run into open areas to gather food (perhaps due to shrinking forests), and then run back to the shelter of the forest (necessitating running on hind legs while holding food in hands), we (perhaps) began our evolution into walking upright.
With this evolution, our pelvises changed, and giving birth became more challenging. Chimp brain size was also increasing, which added to birth-difficulty. So we adapted by giving birth earlier. Therefore human babies are way more helpless and needy than newborns of other primates.
This added neediness made male-involvement very welcome. In chimpanzee society, males customarily give gifts and lavish extra attention on sexually-receptive females. As we evoloved, females with needy young recognized the value in increasing their sexual receptivity, so they could benefit from this extra male attention.
Another factor that contributed to the greater "sexiness" of our species, may have been our upright posture, which made sexual organs more visible, and also made sexual activity more face-to-face and intimate. Sex became a way of enhancing bonds.
In this context, it's easy to see how it could be beneficial, for some people, to form sexual bonds with others of the same sex. These bonds, whether between opposite-sex partners or same-sex partners, greatly increased the likelihood of babies surviving into adulthood.
Thus human sexuality has evolved from being just about procreation, to being more about bonding, meeting one another's needs, and forming partnerships that promote the life and well-being of the couples involved, as well as the life and well-being of everyone in the family and community.